Combustion is a major source of a class of pollutants including oxides of nitrogen, or NOx, (NO or nitric oxide, and NO2 or nitrogen dioxide), which may contribute to acid rain, smog, and ozone depletion. NOx emissions from combustion sources primarily consist of nitric oxide produced during combustion. When utilizing gaseous fuels, combustion processes that decrease the combustion temperature can greatly reduce the production of NO, and, accordingly, can have a significant effect on the overall production of NOx.
Various attempts have been made to re-engineer conventional non-premixed combustion systems to reduce emissions of oxides of nitrogen (NOx). Flames in non-premixed combustion, that is, the combustion process wherein fuel and oxidizer (typically air) mix and burn concurrently, generally emit unacceptable levels of NOx, for example, over 200 parts-per-million (ppm), substantially higher than regulations allow for certain applications. The heating and power generation industries have recognized the need to develop cleaner, premixed combustion systems in which gaseous fuel and oxidizer (typically air) mix prior to burning.
The technical paper, Advanced Catalytic Pilot for Low NOx Industrial Gas Turbines, by Karim, et al., Published by the Proceedings of ASME TURBO EXPO 2002, Jun. 3-6, 2002, Amsterdam, The Netherlands, GT-2002-30083, discloses a catalytic pilot for use in a gas turbine combustor. A catalytic pilot incorporates catalyst-coated tubes to convert part of the fuel gas into combustion products on the surfaces of the tubes. The remainder of the pilot fuel gas and oxidant gas exits the pilot and mixes with fuel gas and oxidant gas from a main swirler to complete the combustion process downstream of the injector. In contrast to traditional pilot injectors, a catalytic pilot allows the operation of the pilot at leaner equivalence ratios. As a result, the inclusion of a catalytic pilot, as opposed to a more traditional pilot injector, provides a reduction in overall NOx levels. Additionally, the presence of a catalyst in the pilot may allow operation of the injector at overall leaner fuel-air ratios, resulting in lower flame temperatures, without combustion driven pressure oscillations.
Accordingly, there exists a need for alternative designs for fuel injectors that address the shortcomings of existing systems and/or that provide reduced NOx emissions. Such designs would be particularly advantageous if they were relatively simple and economical to scale, manufacture and operate.